Erosion damage to gas and steam turbine airfoil components from water droplet impingement and/or hard particle impingement wear during operation results in significant economic losses to the power generation industry. The economic losses are a result of aerodynamic efficiency loss, production downtime, and the costs associated with damaged component refurbishment or replacement. Damage to gas turbine compressor blades by water droplet erosion has become a significant issue since wet compression technologies (SPRITS, Water Evaporate Cooling of inlet air) were introduced to restore turbine efficiency. Water droplet erosion has resulted in many compressor operational issues and has prevented the power generation industry from fully utilizing wet compression technologies. A number of methods have been developed to try to provide erosion resistant coatings on gas and steam turbine components, using various deposition techniques. However, many of these methods still suffer from the above described drawbacks.
Electrospark deposition (ESD) is a pulsed-arc, micro-welding process that uses short-duration, high-current electrical pulses to deposit a consumable electrode material on a conductive workpiece. ESD processes generally involve very high spark frequencies with the spark duration lasting only a few microseconds. ESD generally, and usually requires manual control or preprogramming of the process parameters. Significantly, depositions result in very little heat input because heat is generated during less than 1% of a weld cycle and dissipated during 99% of the cycle. ESD coatings are extremely dense and metallurgically bonded to the workpiece.
Alternative deposition techniques for material repair and protection include high-velocity oxygen fuel (HVOF) thermal spray, physical vapor deposition (PVD), chemical vapor deposition (CVD), and electrolytic hard chrome (EHC) plating. In contrast to most of the above-mentioned techniques, which may produce mechanical or chemical bonds with a workpiece, ESD creates a true metallurgical bond while maintaining the workpiece at or near ambient temperatures. Deposition methods such as sputtering, thermal spay, and plasma vapor deposition form an unreliable physical bond between the coating and component base metal. The coating deposited by these methods readily spalls off from the component surface thereby providing only temporary erosion protection. In addition, the required stoichiometry or tight control of the coating composition is easily violated by unwanted reactions during application of the sputtering, thermal spay, and plasma vapor deposition processes.
One of the distinguishing aspects of ESD, as compared to other arc-welding processes, is that the electrode contacts the surface rather than maintaining a stand-off distance to control the arc. Fusion welding (e.g., laser welding or arc welding) and brazing processes will thermally affect the component causing material property debits, a heat affected zone and unacceptable distortion. Additionally, when using fusion welding or other thermal fusion processes (arc weld, laser, etc.) for depositing a coating it is impossible to achieve tight control of the coating composition. The fusion welding or thermal fusion processes fuse both filler and parent metal which results in a mixture of filler and parent metal in the deposited coating, which prevents tight control of the final coating composition.
A drawback to conventional electrospark deposition devices is that it employs an electrode rod, which is required to have a sharp tip for generating electrical discharges or sparks. When using conventional ESD devices it is almost impossible to produce uniform and high quality coatings on any irregular or highly contoured surface such as the leading edge surface of a blade of a gas turbine.
Therefore a method of modifying and coating steam or gas turbine components using a portable ESD device that allows for the deposition of a compositionally controlled protective coating does not suffer from the above drawbacks is desirable in the art.